This simple analog circuit requires only a handful of components to improve the linearity and/or efficiency of base-station and handset power amplifiers.
Analog approaches may reap huge benefits for digital communications systems. That is the intention of the engineers at Cigma Technologies (Allendale, NJ), who have developed a unique analog predistortion-linearizer (PDL) circuit that promises to make wireless power amplifiers (PAs) at least 50-percent more efficient and consequently much smaller than designs incorporating the best alternative linearization schemes, including feedforward techniques. The solution has been found to be effective with all wireless formats tested, including IS-95 code-division multiple access (CDMA), wideband CDMA (WCDMA), Universal Mobile Telecommunications Services (UMTS), Global System for Mobile Communications (GSM), and Enhanced Data-rate for GSM Evolution (EDGE) formats.
According to Cigma's president and chief operating officer (COO), Young Sohn, "New amplifiers built with this RF predistortion linearizer will be up to 50 percent more compact, will increase efficiency by at least 50 percent, and can be priced at 30 percent less than existing products." Sohn, having spent many fruitful years with software giant Oracle, headed an investment group that acquired Cigma this past year and established two operating divisions: a wireless amplifier group and a test and measurement instrument group (which will continue to operate under the well-established brand name Protek).
The PDL employs proprietary patent-pending Multi-Element RF Predistortion (MERP) technology to achieve the gain and phase control needed to linearize broadband single-channel and multichannel PAs. According to Richard Lisco, Cigma's vice president of engineering, the company's engineers evaluated a variety of techniques for linearization: "We considered ways to make a better linearizer, without using conventional, inefficient feedforward linearization. For example, we looked at distortion feedback and envelope feedback, but all of these approaches are bandwidth limited."
After a period of extensive research, the Cigma engineering team had an analog predistorter working in the laboratory. "The circuit is very simple and amenable to integration, and can be realized with very low-cost components," Lisco notes. The MERP PDL technology can be applied to both base-station amplifiers as well as handsets and cellular telephones. In essence, the PDL circuit consists of two voltage-controlled nonlinear modulators. The outputs of the modulators are summed to generate the desired gain and phase-transfer functions. Linearization is achieved by predistorting the amplitude and phase components of the input signal to the RF PA in a manner to correct for the amplifier gain and phase non-linearity.
Cigma confirms that the circuit has been tested with a number of in-house PA designs as well as PAs from Motorola (Schaumburg, IL) and handset RF IC PA modules, with as much as 15-dB reduction in distortion compared to tests run on the same amplifiers without the PDL circuit. Lisco points out that the PDL can also be used to improve amplifier efficiency, "or to transform a single-carrier amplifier into a multicarrier amplifier." When used in a closed-loop approach, the PDL can deliver an additional 5 to 6 dB in margin. "For example," offers Lisco, "a manufacturer with a 20-W amplifier having a certain level of linearity may be able to achieve 30 W output power with that same level of linearity." The circuit can increase output power for the same distortion performance, or maintain the same output power at a lower distortion level.
The analog linearizer (see figure) is designed for use with low-level input signals from −10 to 0 dBm. It is currently available in three basic versions for a variety of different operating bands. The basic model is the CT-PDL, with versions for 800 to 900 MHz, 1800 to 2100 MHz, and 2400 MHz. Optimally, the device is inserted prior to a PA's driver-amplifier input. The CT-PDLU is a version of the circuit (available in the same frequency bands) with 0-dB insertion loss to ease applications having limited available gain. Finally, the CT-PDLC is a closed-loop, adaptive version of the linearizer circuit complete with microcontroller-based automatic gain control, automatic pre-distorter tuning algorithms to maximize the signal to IMD ratio, and spectrum-monitoring capabilities for remote system provisioning and diagnosis. The complete module includes an RS-232 interface for alarm status and spectrum display. The microcontroller can set gain levels automatically while also simplifying the calibration process, without the need of a pilot tone.
In-house demonstrations and displays performed at the recent CTIA event (New Orleans, LA) have shown the PDLs to achieve better than 50-percent efficiency improvement when used with a multicarrier amplifier operating with four carriers (an operational bandwidth of about 80 MHz). The demonstrations also revealed a better than 10-dB improvement in distortion reduction. The beauty of the adaptive, closed-loop version of the circuit, notes Lisco, is its ability to maintain linearity even under extreme changes in environmental and operating conditions.
The PDL circuitry offers amplifier designers a chance to dramatically improve the performance of their existing single-carrier and multicarrier designs. The analog circuit provides open-loop (nonadaptive) or closed-loop (adaptive) corrections in PA performance that can translate into less power consumed in cellular base stations and more talk time per battery charge for cellular handsets. For a more detailed technical look at this innovative analog linearizer circuit, don't miss the follow-up PDL article in the May issue of Microwaves & RF.